Why is overwatering bad?

joel_bc(z6 BC)March 14, 2011

I know what overwatering can do to houseplants... the thick, yellowing leaves that then begin to fall off the stems. What I don't know are the technicalities involved.

I do know that roots can rot from being over-soaked in water. Is this root rotting the only cause of the ill effects of overwatering? Thanks.

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amccour

Other people probably know more but from my understanding two things tend to happen:

1. Roots die from a lack of oxygen. When they die, they can't take up water, causing a drought response even though the soil is soaked.

2. Rotting roots lead to bacterial and fungal growth which in turn gets into the plant and turns it into mush.

    Bookmark   March 14, 2011 at 7:30PM
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tapla (mid-MI z5b-6a)

Lack of oxygen also affects root function and metabolism. It inhibits the plants ability to grow and take up water AND nutrients. Low oxygen levels also cause several elements, iron and manganese especially, to become electron acceptors. This leads to their precipitation (iron and manganese) and (even though these elements might be present in soils) the plant�s inability to assimilate them. Soggy soils also make Ca difficult or impossible for uptake. Several gasses also form under hypoxic conditions - methane, sulfurous compounds in gaseous forms, and CO2 as a byproduct, none of which are even close to desirable in the rhizosphere (root zone).

Soils that drain freely and remain well aerated, even at container capacity (when the soil if fully saturated) will always offer the best opportunity for your plants to grow nearest to their genetic potential within the limits of other cultural factors. Soils comprised primarily of fine ingredients like peat/coir/sand/topsoil .... have the limiting factor of excess water retention built into them. If you choose to water in small amounts to avoid the root issues associated with overly water-retentive soils, you automatically acquire the build-up of salts that accompanies these soils - unless you regularly make an effort to thoroughly flush accumulating salts from these soils.

In the balance with better plants that are easier to maintain, is the fact that well-aerated and durable soils often require some initial effort, because you have to make them, and you will need to water a little more frequently - a decidedly GOOD thing from the plant's perspective.

In the end, the amount of effort it takes to make your own quality soil, probably ends up just about equaling the effort it takes over time to grow in a poor soil. The primary difference is, only one of the choices ensures the best opportunity for your plants.

Al

    Bookmark   March 14, 2011 at 7:57PM
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cactusmcharris

In succulent plants, the rootball usually rots, followed sometimes by the plant itself. Of course, this can take months and months, so you usually have time to stage an intervention.

    Bookmark   March 14, 2011 at 8:20PM
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joel_bc(z6 BC)

Thanks cactusmcharris, Al, amccour.

Al, that's a very thorough-going look at the factors. Thanks.

I once noted your recommended soil-less growing medium, Al. You said "a 5:1:1 mix of fine pine bark:peat:perlite". That's a very "fast" mix.

All things being simple, I'd probably go with your mix. Instead for most plants I use 2:2:1 of coir:ground-bark:perlite (with a dash of gypsum mixed in). And sometimes I add some poultry grit (rock) to it. A good mix for holding some water over about 4 days, so if my wife and I leave on a vacation for a couple weeks, we can trust a neighbor to water often enough. (You said plants in your mixture will want a good watering every other day.)

But we just purchased some potted young primroses from a nursery, and the "soil" they came in seems so water-retentive that more than half of them are showing signs of too much water, even on a light-watering (and no fertilizer) schedule here at our place. That's what prompted my original post.

    Bookmark   March 15, 2011 at 2:42PM
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tapla (mid-MI z5b-6a)

Try reading this thread about dealing with water-retentive soils. It should offer some tips that will help you cope with those soils if you're noticing issues.

Even in the smallish containers I tend to use, I'm only watering the plants in the gritty mix every 2-4 days, and many are under lights. With a larger volume of soil, you should easily be able to go 4-5 days between waterings - especially if you were to use something like the 5:1:1 mix, which holds quite a bit more water than the gritty mix, but still remains well-aerated and drains very well. Your call - that you understand the concept and know how to work with it is the most important thing.

Al

    Bookmark   March 15, 2011 at 3:23PM
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joel_bc(z6 BC)

Thanks cactusmcharris, Al, amccour.

Al, that's a very thorough-going look at the factors. Thanks.

I once noted your recommended soil-less growing medium, Al. You said "a 5:1:1 mix of fine pine bark:peat:perlite". That's a very "fast" mix.

All things being simple, I'd probably go with your mix. Instead for most plants I use 2:2:1 of coir:ground-bark:perlite (with a dash of gypsum mixed in). And sometimes I add some poultry grit (rock) to it. A good mix for holding some water over about 4 days, so if my wife and I leave on a vacation for a couple weeks, we can trust a neighbor to water often enough. (You said plants in your mixture will want a good watering every other day.)

But we just purchased some potted young primroses from a nursery, and the "soil" they came in seems so water-retentive that more than half of them are showing signs of too much water, even on a light-watering (and no fertilizer) schedule here at our place. That's what prompted my original post.

    Bookmark   March 15, 2011 at 4:55PM
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joel_bc(z6 BC)

Whoops!... I dunno how that posted twice. ??

    Bookmark   March 15, 2011 at 5:00PM
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